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A seismic wave generated by Japan's devastating 2011 magnitude 9.0 Tohoku-Oki earthquake travelled nearly 2,900 kilometres to the boundary of Earth's core, reflected back about 13 minutes later and shifted the entire country eastward by roughly six millimetres, according to a study published in the journal Science.
The movement was far too small for anyone to notice, but Japan's highly sensitive GPS network detected the nationwide shift almost simultaneously. Scientists were unable to explain the unusual signal for 15 years because it did not match the main earthquake, any aftershock or another known geological event. The new research identifies the returning seismic wave as the cause, marking the first documented observation of such a phenomenon.
How a wave travelled nearly 2,900 km beneath Earth's surface
The study was led by Sunyoung Park, assistant professor of geophysical sciences at the University of Chicago, alongside Hiroo Kanamori of the California Institute of Technology and Luis Rivera of the University of Strasbourg.The researchers found that the unexplained GPS signal was caused by an ScS wave, a type of seismic shear wave produced during powerful earthquakes. Unlike the surface waves responsible for much of the shaking people experience, ScS waves travel deep through Earth's solid mantle.
After the 2011 Tohoku-Oki earthquake, one such wave descended nearly 2,890 kilometres until it reached the core-mantle boundary, the boundary separating Earth's rocky mantle from its liquid outer core.Shear waves cannot pass through liquids, so when the ScS wave reached the molten iron and nickel of the outer core, it could not continue its journey. Instead, it reflected off the core-mantle boundary and travelled back towards the surface, much like an echo bouncing off a wall.
The complete round trip covered nearly 5,800 kilometres, making it one of the deepest seismic journeys ever linked to a measurable effect at Earth's surface.Researchers say the exceptional magnitude of the Tohoku-Oki earthquake produced an unusually powerful ScS wave, allowing enough energy to survive the long journey through Earth's interior. By the time it returned to Japan, the wave was still strong enough to influence faults that had already been pushed close to their breaking point by the main earthquake.
Why the wave returned after 13 minutes
Scientists calculated that the journey to the core-mantle boundary and back would take around 13 minutes, matching the unexplained GPS signal recorded across Japan.When the reflected wave arrived, it reached much of the country at nearly the same time. Researchers believe it triggered tiny slips along tectonic plate boundaries that were already under immense stress from the main earthquake. Those small movements permanently shifted Japan eastward by around five to six millimetres.Although the displacement was barely measurable, the combined fault slips are estimated to have released energy comparable to a magnitude 7.5 earthquake.
Why this had never been observed before
Seismologists have studied ScS waves for decades and routinely use them to investigate Earth's deep interior. What had never been observed was one returning with enough energy to leave a permanent mark on Earth's surface.According to the researchers, the Tohoku-Oki earthquake was exceptional because of its enormous magnitude.
The reflected ScS wave recorded across Japan had a peak-to-peak amplitude exceeding one centimetre, making it far stronger than those generated by most earthquakes. Combined with already stressed faults, that unusual energy appears to have been sufficient to trigger additional ground movement.
The GPS signal that puzzled scientists for 15 years
Japan operates one of the world's densest GPS monitoring networks, with more than 1,300 continuously operating stations capable of detecting even the smallest ground movements.About 15 minutes after the earthquake struck, instruments recorded a nearly simultaneous eastward shift across the country. The pattern did not match the main rupture, any known aftershock or a submarine landslide, leaving scientists without a convincing explanation for more than a decade.The new analysis finally links that long-standing anomaly to the returning ScS wave.
How researchers ruled out other possibilities
Before reaching their conclusion, the team evaluated several competing explanations.If the main rupture had continued releasing energy, the movement should have been strongest near the epicentre rather than appearing almost uniformly across Japan. Likewise, neither an unrecorded aftershock nor an underwater landslide could explain both the timing and the nationwide pattern observed by the GPS network.According to the researchers, the reflected ScS wave is the only mechanism that accounts for all of the observations.
The discovery's implications for earthquake science
The findings suggest the effects of Earth's largest earthquakes may extend much deeper and farther than previously understood.Until now, seismic hazards have largely been associated with the main rupture, aftershocks and tsunamis. The study indicates that seismic waves travelling thousands of kilometres through Earth's interior may also be capable of triggering additional fault movement after reflecting from the boundary above the outer core.Researchers now plan to re-examine data from other giant earthquakes, including the 2004 Sumatra-Andaman earthquake, the 1960 Valdivia earthquake in Chile, the 1964 Alaska earthquake and the 2010 Maule earthquake, to determine whether the same mechanism has occurred elsewhere.
What the finding reveals about Earth's hidden interior
The study also highlights how scientists investigate parts of the planet that remain physically inaccessible. The deepest hole ever drilled, Russia's Kola Superdeep Borehole, reached only about 12 kilometres, while Earth's mantle extends to roughly 2,890 kilometres before meeting the liquid outer core.Since humans cannot directly explore these depths, researchers rely on seismic waves generated by powerful earthquakes to understand the planet's internal structure. By showing that a seismic wave travelled to the edge of Earth's core, returned to the surface and produced a measurable effect, the Tohoku-Oki earthquake has provided an unprecedented glimpse into the dynamic connection between Earth's deep interior and its crust.
